After building a torsion box for my lowrider CNC router I discovered I didn't install enough threaded inserts for hold downs.. these are 3d printable ones threaded for 3/8" -16 bolts (I recommend nylon bolts if you can find them).

These are sized for a 4" thick table. To install drill a 5/8" hole all the way through your table and shove this in from the bottom. Use PETG or better filament with 90% infill for strength.

If you need a different length than 4" do NOT resize these.. the threads will no longer work. You'll need to either edit the object or add a spacer under the table.